Holographic photopolymer compositions and elements containing a ring-opening monomer

ABSTRACT

Presence of a ring opening monomer in a photopolymerizable composition improves reflection holograms prepared from the composition.

FIELD OF THE INVENTION

This invention relates to imaging systems in which the imaged layercontains image areas having an index of refraction which is differentfrom that of non-image areas. More particularly this invention relatesto such systems wherein the refractive index image is a reflectionhologram.

BACKGROUND OF THE INVENTION

The term "image recording" is conventionally taken to mean a processwhich produces a spatial pattern of optical absorption in the recordingmedium. Photographic processes are well known examples of this type ofprocess.

In a broader sense, however, the word "image" means a spatial variationof the optical properties of a sample in such a way as to cause adesired modification of a beam of light passing through, or reflectingfrom, the sample. Refractive index images in general and holograms inparticular, which modulate the phase, rather than the amplitude, of thebeam passing through them are usually referred to as phase holograms.Phase holographic image recording systems produce a spatial pattern ofvarying refractive index rather than optical absorption in the recordingmedium and, thus, modulate a beam of light without absorbing it. Thistype of refractive index image formation includes a number of opticalelements or devices, such as holographic lenses, gratings, mirrors, andoptical waveguides, which superficially bear little resemblance toabsorption images.

Holography is a form of optical information storage. The generalprinciples are described in a number of references, e.g., "Photographyby Laser" by E. N. Leith and J. Upatnieks in Scientific American, 212,No. 6,24-35 (June, 1965). A useful discussion of holography is presentedin "Holography", by C. C. Guest, in Encyclopedia of Physical Science andTechnology, Vol. 6, pp. 507-519, R. A. Meyers, Ed., Academic Press,Orlando, Fla., 1987. In brief, the object to be photographed or imagedis illuminated with coherent light (e.g., from a laser) and a lightsensitive recording medium (e.g., a photographic plate) is positioned soas to receive light reflected from the object. This beam of reflectedlight is known as the object beam. At the same time, a portion of thecoherent light is directed to the recording medium, bypassing theobject. This beam is known as the reference beam. The interferencepattern that results from the interaction of the reference beam and theobject beam impinging on the recording medium is recorded in therecording medium. When the processed recording medium is subsequentlyappropriately illuminated and observed at the appropriate angle, thelight from the illuminating source is diffracted by the interferencepattern recorded in the recording medium to reconstruct the wavefrontthat originally reached the recording medium from the object. Thus, thehologram resembles a window through which the virtual image of theobject is observed in full three-dimensional form, complete withparallax.

Holograms that are formed by allowing the reference and object beams toenter the recording medium from the same side are known as transmissionholograms. Interaction of the object and reference beams in therecording medium forms fringes of material with varying refractiveindices which are approximately normal to the plane of the recordingmedium. When the hologram is played back by viewing with transmittedlight, these fringes refract the light to produce the viewed virtualimage. Such transmission holograms may be produced by methods which arewell known in the art, such as disclosed in Leith and Upatnieks, U.S.Pat. Nos. 3,506,327; 3,838,903 and 3,894,787. A diffraction grating isthe simplest possible transmission hologram. It is the hologram of twocoherent plane waves. It can be created by splitting a single laser beamand recombining the beams at the recording medium.

Holograms formed by allowing the reference and object beams to enter therecording medium from opposite sides, so that they are traveling inapproximately opposite directions, are known as reflection holograms.Interaction of the object and reference beams in the recording mediumforms fringes of material with varying refractive indices which are,approximately, in planes parallel to the plane of the recording medium.When the hologram is played back these fringes act as partial mirrorsreflecting incident light back to the viewer. Hence, the hologram isviewed in reflection rather than in transmission. Since the wavelengthselectivity of this type of hologram is very high, white light may beused for reconstruction.

Reflection holograms may be produced by an on-axis method wherein thebeam of coherent radiation is projected through the recording mediumonto an object therebehind. In this instance, the reflected object beamreturns and intersects with the projected beam in the plane of therecording medium to form fringes substantially parallel to the plane ofthe medium. Reflection holograms also may be produced by an off-axismethod wherein a reference beam is projected on one side of therecording medium and an object beam is projected on the reverse side ofthe medium. In this instance the object beam is formed by illuminatingthe object with coherent radiation which has not passed through therecording medium. Rather, the original beam of coherent radiation issplit into two portions, one portion being projected on the medium andthe other portion being directed to project on the object behind themedium. Reflection holograms produced by an off-axis process aredisclosed in U.S. Pat. No. 3,532,406.

A holographic mirror is the simplest possible reflection hologram. It isthe hologram of two coherent plane waves intersecting in a recordingmedium from substantially opposite directions. It can be created bysplitting a single laser beam and recombining the beams at the recordingmedium, or the unsplit laser beam can be projected through the mediumonto a plane mirror therebehind. A set of uniformly spaced fringes areformed that have a sinusoidal-like intensity distribution. The fringesare oriented parallel to the bisector of the obtuse angle between thetwo beams propagating in the recording medium. If the obtuse angle is180° and the beams are normal to the plane of the medium, the fringeswill be parallel to the plane of the medium. If the two beams do notmake equal angles with the normal to the plane of the medium, then thefringes which are formed will be slanted at an acute angle relative tothe plane of the medium. The holographic mirror can be characterized byits wavelength of maximum reflection and by its reflection efficiency,that is, by the percent of incident radiation which is reflected at itswavelength of maximum reflection.

The substantially horizontal fringes which form reflection holograms aremuch more difficult to record than the perpendicular fringes which formtransmission holograms for two reasons. The first reason is the need forhigher resolution, i.e., the need for more fringes per unit length, andthus a closer fringe spacing. Horizontal reflection holograms requireabout 3X to 6X more fringes per unit length than do transmissionholograms. The second reason is the sensitivity of horizontal fringes toshrinkage of the recording medium during exposure. Any shrinkage of therecording medium during exposure will tend to wash out the fringes and,if severe, will prevent a hologram from being formed. This is incontrast to the transmission hologram case, where shrinkage has littleor no effect when the fringes are perpendicular to the plane of themedium, and produces only relatively minor image distortion if thetransmission fringes are slanted more than 45° from the plane of themedium.

A variety of materials have been used to record holograms. Among themore important are: silver halide emulsions, hardened dichromatedgelatin, ferroelectric crystals, photopolymers, photochromics andphotodichroics. Characteristics of these materials are given in VolumeHolography and Volume Gratings, by L. Solymar and D. J. Cook, Chapter10, Academic Press, New York, 1981, pp. 254-304.

Dichromated gelatin is the material most widely used for recordingholograms. This material has become the popular choice because of itshigh diffraction efficiency and low noise characteristics. However,dichromated gelatin has poor shelf life and requires wet processing.Plates must be freshly prepared, or prehardened gelating must be used.Wet processing means that an additional step is required in hologrampreparation and may also cause the hologram to change due to swellingand then shrinkage of the gelating during processing. Shrinkage mayparticularly be a problem when preparing reflection holograms in thatthe shrinkage will alter the wavelength of maximum reflection. Therequirement that plates by freshly prepared each time a hologram is madeplus the problems associated with wet processing make reproducibilityextremely difficult to achieve with dichromated gelatin.

While early holograms where prepared from silver halide or dichromatedcolloids which required several processing steps, photopolymerizableelements have been proposed which require only a single process step.U.S. Pat. No. 3,658,526 to Haugh discloses preparation of stable,high-resolution holograms from solid, photopolymerizable layers by asingle step-process wherein a permanent refractive index image isobtained by a single imagewise exposure of the photopolymerizable layerto actinic radiation bearing holographic information. The holographicimage formed is not destroyed by subsequent uniform actinic exposure,but rather is fixed or enhanced.

Despite the many advantages of the solid photopolymerizable layersproposed by Haugh, reflection holograms produced therefrom have beenpoor at best, with little reflection efficiency. Thus, there is a needfor improved compositions for the preparation of reflection holograms.

SUMMARY OF THE INVENTION

This invention provides storage stable, solid, photopolymerizablecompositions which produce reflection holograms having improvedreflection efficiency. Whereas reflection efficiencies of prior artsolid formulations typically are 10% or less, reflection efficiencies ashigh as 30% can readily be achieved using the compositions of thisinvention. Resulting holograms not only are brighter, but have broaderviewing angles since the improved reflection efficiency permits use of athinner coating of the photopolymer composition.

More particularly, in one embodiment this invention provides asubstantially solid, photopolymerizable composition that forms areflection hologram upon exposure to actinic radiation as the soleprocessing step, said composition consisting essentially of:

(a) 25 to 75% of a solvent soluble, thermoplastic polymeric binder;

(b) 5 to 60% of a monomer capable of addition polymerization, saidmonomer having a boiling point above 100° C. and polymerizing viaring-opening sigmabond cleavage;

(c) 0.1 to 10% of a photoinitiator system that activates polymerizationof said monomer on exposure to actinic radiation;

wherein said percentages are weight percentages of the totalcomposition. As used herein the term "actinic radiation" refers to acoherent light source, such as that produced by a laser.

Preferred monomers contain a ring selected from the group consisting ofcarbocyclic rings of three carbon atoms and heterocyclic rings of up tofive atoms containing up to two heteroatoms, wherein said heteroatomsare selected from the group consisting of nitrogen, oxygen, and sulfur.Vinylcyclopropanes, and especially vinylcyclopropanes which contain anunsubstituted vinyl group (i.e., a H₂ C═CH-- group), attached to thecyclopropane ring, and in which the ring is substituted with one or moreelectron-withdrawing groups, are preferred. Ethyl1-acetyl-2-vinyl-1-cyclopropane carboxylate, ethyl1-benzoyl-2-vinyl-1-cyclopropane carboxylate, and ethyl2-vinylcyclopropane-1,1-dicarboxylate are particularly useful monomers.

In a preferred mode the initiator system is a 2,4,5-triphenylimidazolyldimer, or a mixture of dimers, and a hydrogen donor, sensitized by avisible sensitizer. Preferred 2,4,5-triphenylimidazolyl dimers includeCDM-HABI, o-Cl-HABI and TCTM-HABI. Sensitizers that may be used toadvantage include the bis(p-dialkylaminobenzylidine) ketones disclosedin Baum and Henry, U.S. Pat. No. 3,652,275 and the arylidene arylketones disclosed in Dueber, U.S. Pat. No. 4,162,162, as well as in U.S.Pat. Nos. 4,268,667 and 4,351,893.

The photopolymerizable composition typically will be coated or laminatedonto a suitable substrate, such as polyethylene terephthalate, to forman element that may be imaged to form a hologram.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the experimental arrangement used to prepareholographic mirrors.

FIG. 2 illustrates the experimental arrangement used to prepareholographic gratings.

DETAILED DESCRIPTION OF THE INVENTION

The improved photopolymerizable compositions of this invention aresubstantially solid and are typically used as a layer applied to apermanent substrate. The photopolymerizable layer is a thermoplasticcomposition which, upon exposure to actinic radiation, forms polymers ofhigher molecular weight that change the refractive index and rheologicalcharacter of the composition. Free radical addition polymerizationand/or crosslinking of a compound containing one or more ethylenicallyunsaturated groups, usually in a terminal position, hardens andinsolubilizes the composition. The sensitivity of the photopolymerizablecomposition is enhanced by the photoinitiator system that may contain acomponent that sensitizes the composition to practical radiationsources, e.g., visible light.

While the photopolymerizable layer is a solid sheet of uniform thicknessit is composed of at least three major components: a substantiallysolid, solvent soluble, thermoplastic polymeric binder; at least onemonomer capable of addition polymerization, said monomer having aboiling point above 100° C. and polymerizing via ring-opening sigma-bondcleavage; and a photoinitiator system activatible by actinic radiation.Although the layer is a solid composition, components may interdiffusebefore, during and after imaging exposure until they are fixed ordestroyed by a final uniform treatment, usually by a further uniformexposure to actinic radiation or by thermal treatment at elevatedtemperatures. Interdiffusion may be further promoted by incorporationinto the composition of an otherwise unreactive plasticizer. Typically,the composition contains a liquid monomer, but it may contain solidmonomer components, either individually or in combination with one ormore liquid monomers, which are capable of interdiffusing in the solidcomposition and reacting to form a polymer or copolymer with arefractive index shifted from that of the preformed polymeric material.

Monomers

Despite the many advantages of the solid photopolymerizable layersdisclosed in Haugh, U.S. Pat. No. 3,658,526, reflection hologramsproduced therefrom have been poor at best, with little or no reflectionefficiency. These layers contain addition-polymerizable, nongaseous,ethylenically unsaturated monomers capable of forming high polymers byfree radical initiated, chain-propagating addition polymerization; afree radical generating system activatible by actinic radiation; and, inthe preferred case, a macromolecular organic binder that is solid at 50°C. and compatible with said monomer. The preferred binder is celluloseacetate butyrate.

In the compositions of the instant invention, monomers are selectedwhich polymerize via ring-opening sigma-bond cleavage. When used instable, solid, photopolymerizable compositions, these monomers producebright and sharp reflection holograms with good reflection efficiency.Monomers useful in the practice of this invention are those having aboiling point above 100° C. and which polymerize via a ring-openingsigma-bond cleavage. These monomers may be liquid, or they may be solidif the solid monomer can be dissolved in a liquid monomer or monomersand/or a liquid plasticizer or plasticizers or mixtures thereof.

Ring-opening polymerizations has been discussed in Ring-OpeningPolymerization, K. J. Ivin and T. Saegusa, ed., Elsevier, New York,1984, especially Chapter 1, "General Thermodyanamic and MechanisticAspects of Ring-opening Polymerization" by K. J. Ivin and T. Saegusa, pp1-82, and Chapter 2, "Ring-opening Polymerizations via Carbon-CarbonSigma-Bond Cleavage", by H. K. Hall, Jr., and L. G. Snow, pp. 83-119; byW. J. Bailey et al., J. Macromol. Sci.-Chem., A21, 1611-1639, 1984; andby Cho and K. -D. Ahn, J. Poly. Sci., Poly. Lett. Ed., 15, 751-753,1977. Polycyclic ring opened polymers are discussed in Bailey, U.S. Pat.No. 4,387,215. As will be appreciated by those skilled in the art, asigma-bond is a single bond, i.e., one which, to a first approximation,involves only one pair of shared electrons.

Monomers that may be selected are those in which the ring which isopened on polymerization, selected from the group consisting ofcarbocyclic rings with seven or fewer carbon atoms and heterocyclicrings of seven or fewer atoms containing up to two heteroatoms whereinsaid heteroatoms are selected from the group consisting of nitrogen,oxygen, and sulfur. Suitable monomers, which can be used as the solemonomer or in combination with other monomers of this type or which canbe mixed with conventional monomers and/or plasticizers, include:vinylcyclopropanes, such as 1,1-dicyano-2-vinylcyclopropane,1,1-dichloro-2-vinylcyclopropane, diethyl2-vinylcyclopropane-1,1-dicarboxylate (EVCD), ethyl1-acetyl-2-vinyl-1-cyclopropane carboxylate (EAVC), ethyl1-benzoyl-2-vinyl-1-cyclopropane carboxylate (EBVC) and the like;bicyclobutanes, especially those which contain one or moreelectron-withdrawing groups at the bridgehead positions, such as,dimethyl bicyclobutane-1,3-dicarboxylate, and the like; tetramethylenedisulfide; spiro-di-o-xylylene; unsaturated o-carbonates, such as2-methylene-1,3-dioxolane; and unsaturated spiro o-carbonates, such as3,9-bis(methylene)-1,5,7,11-tetraoxaspiro[5,5]undecane.Electronwithdrawing groups that promote sigma bond cleavage aresubstituents such as carboethoxy, cyano, acetyl, benzoyl, and the like,which have positive substituent constants, or rho values, as measured bythe well-known Hammett equation (see, for example, Physical OrganicChemistry, by L. P. Hammett, McGraw-Hill, New York, 1940, Chapter 7, pp.184-227, or Physical Organic Chemistry, by J. Hine, McGraw-Hill, NewYork, 2nd. ed., 1962, Chapter 4, pp. 81-103).

Preferred monomers contain a ring which is selected from the groupconsisting of carbocyclic rings of three carbon atoms and heterocyclicrings of up to five atoms containing up to two heteroatom wherein saidheteroatoms are selected from the group consisting of nitrogen, oxygen,and sulfur. Vinylcyclopropanes, and especially vinylcyclopropanes whichcontain an unsubstituted vinyl group (i.e., a H₂ C═CH-- group) attachedto the cyclopropane ring, and in which the ring, is substituted with oneor more electron-withdrawing groups, are preferred. Particularly usefulmonomers are: diethyl 2-vinylcyclopropane-1,1-dicarboxylate (EVCD);ethyl 1-acetyl-2-vinyl-1-cyclopropane carboxylate (EAVC); and ethyl1-benzoyl-2-vinyl-1-cyclopropane carboxylate (EBVC); which have thefollowing structures: ##STR1## EVCD R=--CO₂ CH₂ CH₃ EAVC R=--COCH₃

EBVC R=--COC₆ H₅

Binder

The binder serves as a containing medium for the monomer,photoinitiator, and other components prior to exposure; provides thebase line refractive index; and, after exposure, contributes to thephysical and refractive index characteristics needed for the reflectionhologram or refractive index image formed. Cohesion, adhesion,flexibility, miscibility, tensile strength, in addition to index ofrefraction, are some of the many properties which determine if thebinder is suitable for use in a refractive index recording medium.

Useful binders are performed macromolecular polymeric or resinmaterials, typically having a molecular weight above 1000, including thefollowing: polymers and copolymers of acrylate and alpha-alkyl acrylateesters, e.g., polymethyl methacrylate and polyethyl methacrylate;polymers and copolymers of vinyl esters and their hydrolysis and partialhydrolysis products, e.g., polyvinyl acetate, polyvinylacetate/acrylate, polyvinyl acetate/methacrylate and hydrolyzedpolyvinyl acetate; ethylene/vinyl acetate copolymers; styrene polymersand copolymers, with, e.g., maleic anhydride, or acrylate andmethacrylate esters; vinylidene chloride copolymers, e.g., vinylidenechloride/acrylonitrile, vinylidene chloride/methacrylate, and vinylidenechloride/vinyl acetate; vinyl chloride polymers and copolymers, e.g.,vinyl chloride/acetate; saturated and unsaturated polyurethanes;synthetic rubbers, e.g., butadiene/acrylonitrile,acrylonitrile/butadiene/styrene,methacrylate/acrylonitrile/butadiene/styrene copolymers,2-chlorobutadiene-1,3 polymers, chlorinated rubber, andstyrene/butadiene/styrene and styrene/isoprene/styrene block copolymers;poly(ethylene imine); polyepoxides having average molecular weights fromabout 4,000 to 1,000,000; copolyesters, e.g., those prepared from thereaction product of a polymethylene glycol of the formula HO(CH₂)_(n)OH, where n is an integer of from 2 to 10 inclusive, with (1)hexahydroterephthalic, sebacic and terephthalic acids, (2) terephthalic,isophthalic and sebacic acids, (3) terephthalic and sebacic acids, (4)terephthalic and isophthalic acids, or (5) mixtures of copolyestersprepared from said glycols and (i) terephthalic, isophthalic and sebacicacids and (ii) terephthalic, isophthalic, sebacic and adipic acids;nylons or polyamides, e.g., N-methoxymethyl polyhexamethylene adipamide;cellulose esters, e.g., cellulose acetate, cellulose acetate succinateand cellulose acetate butyrate; cellulose ethers, e.g., methylcellulose, ethyl cellulose and benzyl cellulose; polycarbonates;polyvinyl acetals, e.g., polyvinyl butyral, polyvinyl formal;polyformaldehydes; poly N-vinyl carbazole and copolymers thereof; andcarbazole containing polymers such as those disclosed by H. Kamogawa etal., J. Poly. Sci.: Poly. Chem. Ed., 18, 9-18, 1979.

In the preferred stable, solid, photopolymerizable compositions adaptedfor the preparation of holograms, the binder and the monomer areselected so that either the binder or the monomer contains one or moremoieties selected from the group consisting of substituted orunsubstituted phenyl, phenoxy, naphthyl, naphthyloxy, and heteroaromaticgroups containing up to three aromatic rings; chlorine; and bromine. Theother component is substantially free of these specified moieties. Inthe instance when the monomer contains these moieties, thephotopolymerizable system hereinafter is identified as a "MonomerOriented System" and when the polymeric material contains thesemoieties, the photopolymerizable system hereinafter is identified as a"Binder Oriented System".

The monomer of a Monomer Oriented System of this invention is a compoundcapable of addition polymerization via carbon-carbon sigma-bond cleavageand having a boiling point above 100° C. which contains one or moremoieties taken from the group consisting of a substituted orunsubstituted phenyl, phenoxy, naphthyl, naphthyloxy, and heteroaromaticgroups containing up to three aromatic rings; chlorine; and bromine. Themonomer contains at least one such moiety and may contain two or more ofthe same or different moieties of the group. A preferred monomer of thisinvention for use in the Monomer Oriented System is ethyl1-benzoyl-2-vinyl-1-cyclopropane carboxylate (EBVC).

The solvent soluble polymeric material or binder of the Monomer OrientedSystem in substantially free of substituted or unsubstituted phenyl,phenoxy, naphthyl, naphthyloxy, and heteroaromatic groups containing upto three aromatic rings; chlorine; and bromine. Candidate binders ofthis class, which are solvent soluble, thermoplastic polymers, which canbe used alone, or in combination with one another and include thefollowing: acrylate and alpha-alkyl acrylate ester and acid polymers andinterpolymers e.g., polymethyl methacrylate and polyethyl methacrylate;polyvinyl esters, e.g., polyvinyl acetate, polyvinyl acetate/acrylate,polyvinyl acetate/methacrylate and hydrolyzed polyvinyl acetate;ethylene/vinyl acetate copolymers; saturated and unsaturatedpolyurethanes; butadiene and isoprene polymers and copolymers and highmolecular weights polyethylene oxides of polyglycols having averagemolecular weights from about 4,000 to 1,000,000; epoxides, e.g.,epoxides containing acrylate or methacrylate groups; polyamides, e.g.,N-methoxymethyl polyhexamethylene adipamide; cellulose esters, e.g.,cellulose acetate, cellulose acetate succinate and cellulose acetatebutyrate; cellulose ethers, e.g., methyl cellulose, and ethyl cellulose,polycarbonates; polyvinyl acetal, e.g., polyvinyl butyral, polyvinylformal; polyformaldehydes. Acid containing polymers and copolymersfunctioning as suitable binder include those disclosed in U.S. Pat. No.3,458,311 and in U.S. Pat. No. 4,273,857 as well as the amphotericpolymeric binders disclosed in U.S. Pat. No. 4,293,635.

Particularly preferred binders for use in the Monomer Oriented System ofthis invention are cellulose acetate butyrate polymers; acrylic polymersand inter polymers including polymethyl methacrylate, methylmethacrylate/methacrylic acid and methyl methacrylate/acrylic acidcopolymers, terpolymers of methylmethacrylate/C₂ -C₄ alkyl acrylate ormethacrylate/acrylic or methacrylic acid; polyvinyl acetate; polyvinylacetal; polyvinyl butyral; and polyvinyl formal; as well as mixturesthereof.

The monomer of a Binder Oriented System of this invention is a compoundcapable of addition polymerization via carbon-carbon sigma-bond cleavageand having a boiling point above 100° C. which is substantially free ofmoieties taken from the group consisting essentially of substituted orunsubstituted phenyl, phenoxy, naphthyl, naphthyloxy, and heteroaromaticgroups containing up to three aromatic rings; chlorine; and bromine.Preferred monomers of this invention for use in the Binder OrientedSystem are diethyl 2-vinylcyclopropane-1,1-dicarboxylate (EVCD) andethyl 1-acetyl-2-vinyl-1 cyclopropane carboxylate (EAVC).

The solvent soluble polymeric material or binder of the Binder OrientedSystem contains in its polymeric structure moieties taken from the groupconsisting essentially of substituted or unsubstituted phenyl, phenoxy,naphthyl, naphthyloxy, and heteroaromatic groups containing up to threearomatic rings; chlorine; and bromine. The moieties may form part of themonomeric units which constitute the polymeric binder or may be graftedonto a preformed polymer or interpolymer. The binder of this type may bea homopolymer or it may be an interpolymer of two or more separatemonomeric units wherein at least one of the monomeric units contains oneof the moieties identified above.

Candidate binders of this class which are solvent soluble, thermoplasticpolymers or interpolymers can be used alone, or in combination with oneanother include the following: polystyrene polymers and copolymers,e.g., with maleic anhydride, acrylic acid, methacrylic acid and estersthereof; vinylidene chloride copolymers, e.g., vinylidenechloride/acrylonitrile; vinylidene chloride/methacrylate and vinylidenechloride/vinyl acetate copolymers; polyvinyl chloride and copolymers,e.g., polyvinyl chloride/acetate;methacrylate/acrylonitrile/butadiene/styrene copolymers,2-chlorobutadiene-1,3 polymers, chlorinated rubber, andstyrene/butadiene/styrene, styrene/isoprene/styrene block copolymers;copolyesters, e.g., those prepared from the reaction product of apolymethylene glycol of the formula HO(CH₂)_(n) OH, where n is a wholenumber 2 to 10 inclusive, and (1) hexahydroterephthalic, sebacic andterephthalic acids, (2) terephthalic, isophthalic and sebacic acids, (3)terephthalic and sebacic acids, (4) terephthalic and isophthalic acids,and (5) mixtures of copolyesters prepared from said glycols and (i)terephthalic, isophthalic and sebacic acids and (ii) terephthalic,isophthalic, sebacic and adipic acids; cellulose ethers, e.g., ethylbenzyl cellulose; poly N-vinyl carbazole and copolymers thereof; andcarbazole containing polymers such as those disclosed by H. Kamogawa etal., J. Poly. Sci.: Poly. Chem. Ed., 18, 9-18, 1979.

Particularly preferred binders for use in the Binder Oriented Systeminclude polystyrene, poly(styrene/acrylonitrile), poly(styrene/methylmethacrylate), and polyvinyl benzal as well as admixtures thereof.

Initiator Systems

By "actinic radiation" is meant radiation from a coherent light source,such as a laser, which is active to produce the free-radicals necessaryto initiate polymerization of the monomeric material. The initiatorsystem comprises one or more compounds which directly furnishfree-radicals when activated actinic radiation. It can also comprise aplurality of compounds, one of which yields the free-radicals afterhaving been caused to do so by another compound, or sensitizer, which isactivated by the radiation. Photoinitiator systems useful in practicingthis invention typically will contain a photoinitiator and a sensitizerwhich extends the spectral response into regions having special utility,e.g., the near ultraviolet region and the visible and near infraredspectral regions where lasers emit.

A large number of free-radical generating compounds can be utilized toadvantage. Redox systems, especially those involving dyes, e.g., RoseBengal/2-dibutylaminoethanol, may be used. Photoreducible dyes andreducing agents such as those disclosed in U.S. Pat. Nos. 2,850,445;2,875,047; 3,097,096; 3,074,974; 3,097,097; 3,145,104; and 3,579,339; aswell as dyes of the phenanzine, oxazine, and quinone classes can be usedto initiate photopolymerization. A useful discussion of dye sensitizedphotopolymerization can be found in "Dye Sensitized Photopolymerization"by D. F. Eaton in Adv. in Photochemistry, Vol. 13, D. H. Volman, G. S.Hammond, and K. Gollinick, eds., Wiley-Interscience, New York, 1986, pp.427-487.

Preferred initiator systems are 2,4,5-triphenylimidazolyl dimers withchain transfer agents, or hydrogen donors, and mixtures thereof asdescribed in U.S. Pat. Nos. 3,427,161; 3,479,185; 3,549,367; 4,311,783;4,622,286; and 3,784,557, sensitized by visible sensitizers. Useful2,4,5-triarylimidazolyl dimers are disclosed in Baum et al. U.S. Pat.No. 3,652,275 column 5, line 44 to column 7, line 16. Preferred2,4,5-triphenylimidazolyl dimers include CDM-HABI, i.e.,2-(o-chlorophenyl)-4,5-bis(m-methoxyphenyl)-imidazole dimer; o-Cl-HABI,i.e., 1,1'-biimidazole, 2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenyl-; and TCTM-HABI, i.e.,1H-imidazole, 2,5-bis(o-chlorophenyl)-4-[3,4-dimethoxyphenyl]-, dimer,each of which is typically used with a hydrogen donor.

Sensitizers useful with these photoinitiators include methylene blue andthose disclosed in U.S. Pat. Nos. 3,554,753; 3,563,750; 3,563,751;3,647,467; 3,652,275; 4,162,162; 4,268,667; 4,351,893; 4,454,218;4,535,052; and 4,565,769. A preferred group of sensitizers include thebis(p-dialkylaminobenzylidine) ketones disclosed in Baum et al., U.S.Pat. No. 3,652,275 and the arylyidene aryl ketones disclosed in Dueber,U.S. Pat. No. 4,162,162, as well as in U.S. Pat. Nos. 4,268,667 and4,351,893. Useful sensitizers are listed in Dueber, U.S. Pat. No.4,162,162 column 6, line 1 to line 65. Particularly preferredsensitizers include the following: DBC, i.e., cyclopentanone;2,5-bis-[4-(diethylamino)-2-methylphenyl]methylene]-; DEAW, i.e.,cyclopentanone, 2,5-bis[4-(diethylamino)phenyl]methylene]-;dimethoxy-JDI, i.e., 1-inden-1-one,2,3-dihydro-5,6-dimethoxy-2-[(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-9-yl)methylene]-,and JAW, i.e., cyclopentanone,2,5-bis[(1H,5H-benzo[i,j]quinolizin-1-yl)methylene]-, which have thefollowing structures: ##STR2##

Hydrogen donor compounds useful as chain transfer agents in thephotopolymer compositions include: 2-mercaptobenzoxazole,2-mercaptobenzothiazole, 4-methyl-4H-1,2,4,triazole-3-thiol, and thelike; as well as various types of compounds, e.g., (a) ethers, (b)esters, (c) alcohols, (d) compounds containing allylic or benzylichydrogen, (e) acetals, (f) aldehydes, and (g) amides as disclosed incolumn 12, lines 18 to 58 of MacLachlan, U.S. Pat. No. 3,390,996. Othersuitable hydrogen donor compounds, which are preferred for compositionswhich contain N-vinyl carbazole monomer, are5-chloro-2-mercaptobenzothiazole; 2-mercaptobenzothiazole;1H-1,2,4-triazole-3-thiol; 6-ethoxy-2-mercaptobenzothiazole;4-methyl-4H-1,2,4-triazole-3-thiol; 1-dodecanethiol; and mixturesthereof.

Other Components

The solid photopolymerizable compositions of this invention may containa plasticizer to enhance the refractive index modulation of the imagedcomposition. Plasticizers of this invention may be used in amountsvarying from about 2% to about 25% by weight of the compositionpreferably 5 to about 15 wt.%. Suitable plasticizers include triethyleneglycol, triethylene glycol diacetate, triethylene glycol dipropionate,triethylene glycol dicaprylate, triethylene glycol dimethyl ether,triethylene glycol bis(2-ethylhexanoate), tetraethylene glycoldiheptanoate, poly(ethylene glycol), poly(ethylene glycol) methyl ether,isopropylnaphthalene, diisopropylnaphthalene, poly(propylene glycol),glyceryl tributyrate, diethyl adipate, diethyl sebacate, dibutylsuberate, tributyl phosphate, tris(2-ethylhexyl) phosphate, Brij® 30[C₁₂ H₂₅ (OCH₂ CH₂)₄ OH], and Brij® 35 [C₁₂ H₂₅ (OCH₂ CH₂))₂₀ OH].Particularly preferred plasticizers for use in these systems aretriethylene glycol dicaprylate and tetraethylene glycol diheptanoate.

Other plasticizers that yield equivalent results will be apparent tothose skilled in the art, and may be employed in accordance with theinvention. In cases in which a mixture of a solid and a liquid monomerare present, it will also be appreciated that plasticizer may besubstituted for some or all of the liquid monomer, provided that themixture of plasticizer and monomer remains liquid. It will also beappreciated that a mixture of plasticizer and solid monomer may be used,provided that the mixture of plasticizer and monomer remains liquid.

Other conventional components, in addition to those described above, canpresent in the compositions and elements of this invention in varyingamounts. Such components include: optical brighteners, ultravioletradiation absorbing material, thermal stabilizers, and release agents.

Optical brighteners useful in the process of the invention include thosedisclosed in Held, U.S. Pat. No. 3,854,950. A preferred opticalbrightener is 7-(4'-chloro-6'-diethylamino-1', 3', 5'-triazine-4'-yl)amino 3-phenyl coumarin. Ultraviolet radiation absorbing materialsuseful in the invention are also disclosed in Held, U.S. Pat. No.3,854,950.

Useful thermal stabilizers include: hydroquinone, phenidone,p-methoxyphenol, alkyl and aryl-substituted hydroquinones and quinones,tert-butyl catechol, pyrogallol, copper resinate, naphthylamines,beta-naphthol, cuprous chloride, 2,6-di-tert-butyl p-cresol,phenothiazine, pyridine, nitrobenzene, dinitrobenzene, p-toluquinone andchloranil. The dinitroso dimers described in Pazos, U.S. Pat. No.4,168,982, are also useful. Normally a thermal polymerization inhibitorwill be present to increased stability in the storage of thephotopolymerizable composition. A preferred reversible thermalstabilizer is TAOBN, i.e.,1,4,4-trimethyl-2,3-diazobicyclo(3.2.2)-non-2-ene-2,3-dioxide.

Compounds which have been found useful as release agents may also beincorporated in film compositions such as are described in Bauer, U.S.Pat. No. 4,326,010. A preferred release agent is polycaprolactone.

Exposure/Evaluation

In the process of this invention, reflection holograms are preparedusing a substantially solid, photopolymerizable recording medium whichcomprises a transparent, dimensionally stable substrate having thereon athin layer, typically between about 10 and 100 micrometers thick, of aunique, substantially solid, photopolymerizable composition.

In the instance of the "in-line" or "on-axis" mode of the process, auniform, coherent beam of laser radiation, e.g., a collimated 488 nmargon-ion laser beam, is projected onto a first surface of thephotopolymerizable layer typically at an angle of from 0° up to 70° fromthe normal to the plane of the layer. The collimated beam functions inpart as a "reference beam" while a portion of the collimated beam whichis transmitted through the layer and transparent substrate illuminatesan object behind the layer. The portion of the transmitted laserradiation which is reflected by the object forms an "object beam" whichprojects back onto the rear second surface of the layer and interactswith the reference beam in the plane of the layer to form fringes whichare oriented parallel to the bisector.

For the purpose of evaluating materials useful in the processes of thisinvention holographic mirrors are prepared and reflection efficiency andwavelength of maximum reflection are determined. A film element isprepared comprising, in order: a 0.1 mm thick clear polyethyleneterephthalate film support; a photopolymerizable layer between about0.01 to 0.05 mm thick, and polyethylene terephthalate cover sheet.

Coated film is cut into uniform sections, the cover sheet is removed,and the film then mounted by hand laminating the tacky coating directlya glass plate with a hand roller. Even though the layer is solid itssurface typically is tacky and adheres readily to the glass surface. Inthose instances where tack is absent, heat and/or pressure may be usedto laminate the photopolymerizable layer to the glass substrate surface.Typically the film support is left in place on the laminate and servesto protect the layer during exposure and handling operations.

Holographic mirrors are formed by actinic exposure at the intersectionof two counterpropagating beams of an argon-ion laser operating at 488nm, TEM₀₀ mode. This is achieved using the system illustrated in FIG. 1.In the system an argon ion laser (10) operating at 488 nm and TEM₀₀produces a laser beam (12) which is controlled by a shutter (14). Thelaser beam (12) is directed by a mirror (16) into a beam splitter (18)wherein the beam is divided into two equal beam segments (20). Each beamsegment (20) passes through a microscope objective (22), pinhole(spacial filter) (24), and collimating lens (26) to produce an expanded,collimated beam (28). Each expanded, collimated beam (28) is reflectedby a mirror (36) to converge in the plane of photopolymerizable layer(32) approximately normal to the plane of the photopolymerizable layer(32). The photopolymerizable layer (32) is mounted on a glass plate (34)and protected by a polyethylene terephthalate film support (30).

For comparison with compositions which contain addition polymerizablemonomers which do not polymerize via carbon-carbon sigma-bond cleavage,transmission gratings were prepared and the diffraction efficiency ofeach determined. This measurement is achieved using the 30° holographicgrating system illustrated in FIG. 2. In this system an argon ion laser(10) operating at 488 nm and TEM₀₀ produces a laser beam (12) which iscontrolled by a shutter (14). The laser beam (12) is directed by mirrors(16) into a beam splitter (18) wherein the beam is divided into twoapproximately equal beam segments (20). Each beam segment (20) passesthrough a microscope objective (22), pinhole (spatial filter) (24) andcollimating lens (26). The expanded collimated beams (28) are eachreflected by a mirror (36) to converge in the plane of glass mountedsample (32) to subtend an angle of 30° whose bisector is normal to theplane of the sample (32) so as to form a grating hologram. Thephotopolymerizable layer (32) is mounted on a glass plate (34) andprotected by a polyethylene terephthalate film support (30). Gratingformation is measured in real time by passing a 632.8 nm beam (40) froma He:Ne laser (38) through the center of the exposure area at the Braggangle and the intensity of the laser beam (42) diffracted by the sample(32) is monitored with a detector (44).

After holographic mirrors or gratings are recorded, the film samples areoverall exposed to a mixture of ultraviolet and visible light. The filmsupport and attached exposed photopolymerizable layer are then removedfrom the glass plate and transmission spectra of the unprocessedholographic mirrors are recorded at 400-500 nm using a conventionalspectrophotometer. Maximum reflection efficiency is measured from thetransmission spectra.

Reflection efficiencies using prior art solid formulations typically are10% or less whereas the compositions of this invention typically willachieve reflection efficiencies higher than 10%, and as high asapproximately 30%, or higher. Because of the excellent efficiencies thatare achieved, less material is needed to produce an acceptable hologram.

Substrates/Coating

The improved photopolymerizable compositions of this invention aresubstantially solid and are typically used as a layer applied to apermanent substrate. The composition may be directly coated onto thesubstrate by any conventional method or may be laminated thereto as astorage stable, preformed element comprising the photopolymerizablelayer releasably adhered to a temporary support film which isdimensionally stable and, preferably, transparent to actinic radiation,e.g., of polyethylene terephthalate. The other side of the supportedphotopolymerizable layer may have a temporary protective coversheetreleasably adhered thereto, e.g., polyethylene, polypropylene, etc.Typically the coversheet has the weaker adherence to thephotopolymerizable layer and the permanent substrate has the strongeradherence. Conventional intermediate layers or coatings may be used tofacilitate the adhesive and/or release characteristics needed for thepreformed element.

Composition

Amounts of ingredients in the photopolymerizable compositions willgenerally by within the following percentage ranges based on totalweight of the photopolymerizable layer: monomer, 5-60%, preferably15-50%; initiator 0.1-10%, preferably 1-5%: binder, 25-75%, preferably45-65%; plasticizer, if present, 2-25%, preferably 5-15%; otheringredients 0-5%, preferably 1-4%.

Syntheses

The synthesis of vinylcyclopropanes is discussed in "Ring-OpeningPolymerizations via Carbon-Carbon Sigma-Bond Cleavage", by H. K. Hall,Jr., and L. G. Snow, in Ring-Opening Polymerization, K. J. Ivin and T.Saegusa, ed., Elsevier, New York, 1984, Chapter 2, pp. 84-85. Vinylcyclopropane has been prepared from methylcyclopropyl carbinol by acidicdehydration (Van Volkenburgh et al., J. Am. Chem. Soc., 71, 3595, 1949).

In addition to synthetic procedures similar to the one mentioned above,two other general methods are used to prepare vinylcyclopropanes. Theaddition of carbenes to butadiene, as described, for example, by T.Takahashi, J. Poly. Sci., A-1, 6, 403, 1968, constitutes one suchmethod. A modification of this method is described by Buchert andReissig, Tetrahedron Lett., 29, 2319, 1988. The second general method isused to prepare vinylcyclopropanes ring-substituted with twoelectron-withdrawing groups. These compounds are prepared by reaction ofsodium diethylmalonate (or a related compound) with 1,4-dibromo-2-buteneas described, for example, by R. W. Kierstead, et al., J. Chem. Soc.,3610-21 (1952). The synthesis of EAVC and of EBVC by this method isdescribed in Examples 1 and 2, respectively.

Industrial Applications

The compositions and processes of this invention are used in thepreparation of reflection holograms. Reflection holograms can be used indisplays as, for example, in advertising or packaging; in securityapplications as, for example, on credit cards, bank notes, lotterytickets, and the like; for information storage; and for the preparationof holographic optical elements, i.e., holographic mirrors.

Holographic mirrors have certain advantages over conventional mirrors:(1) they can be produced by a photographic process making thempotentially low cost in mass production, (2) the optical configurationis independent of the substrate configuration, (3) they can bespectrally sensitive, performing as narrow band rejection filters, and(4) the physical weight can be insignificant in comparison to that ofconventional optics. Important application of holographic mirrorsinclude holographic notch filters and head-up displays.

A notch filter rejects a selected narrow band of radiation and providesmaximum transmission outside the selected band. Holographic notchfilters provide protection against laser radiation for eyes and forinstruments.

A head-up display is a form of optical combiner, i.e., a dual functionoptical element which simultaneously performs as an optical window(which transmits a nearly undistorted transmitted image) and as ananalog of a conventional mirror or lens. A head-up display is comprisedof a holographic mirror mounted in front of an observer. Wheninformation is projected onto the mirror at the wavelength which theholographic mirror reflects, the observer see the information projectedon the mirror. However, the observer is able to see the outside worldthrough the mirror since the holographic mirror reflects only a narrowband of radiation. Head-up displays are used in aircraft and have beenproposed for use in automobiles.

The advantageous properties of this invention can be observed byreference to the following examples which illustrate, but do not limit,the invention.

                  EXAMPLES                                                        ______________________________________                                        Glossary of Chemical Names                                                    ______________________________________                                        BHT     Butylated hydroxytoluene; 2,6-Di-tert-butyl-4-                                methylphenol; CAS 128-37-0                                            CAB     Cellulose acetate butyrate, Eastman type 531-1;                               CAS 9004-36-8                                                         DEAW    Cyclopentanone, 2,5-bis[[4-                                                   (diethylamino)phenyl]methylene]-;                                             CAS 38394-53-5                                                        EAVC    Ethyl 1-acetyl-2-vinyl-1-cyclopropane carboxylate                     EBVC    Ethyl 1-benzoyl-2-vinyl-1-cyclopropane carboxylate                    EVCD    Ethyl 2-vinylcyclopropane-1,1-dicarboxylate                           MBO     2-Mercaptobenzoxazole; 2-Benzoxaxolethiol;                                    CAS-2382-96-9                                                         MMT     4-Methyl-4H-1,2,4-triazole-3-thiol; CAS 24854-43-1                     -o-Cl-HABI                                                                           1,1'-Biimidazole, 2,2'-bis[ -o-chlorophenyl]-4,5',5,5'-                       tetraphenyl-; CAS 1707-68-2                                           POEA    2-Phenoxyethyl acrylate; CAS 48145-04-6                               PS-AN   72:25 poly(styrene/acrylonitrile)                                     PS-MMA  70:30 Poly(styrene/methyl methacrylate)                               PVB     Poly(vinly butyral), M.W. 36,000; CAS 63148-65-2                      TDA     Triethyleneglycol diacrylate; CAS-1680-21-3                           TDC     Triethyleneglycol dicaprylate; CAS-106-10-5                           ______________________________________                                    

GENERAL PROCEDURES

Sample Preparation

Coating solutions without visible sensitizer, DEAW, were prepared underyellow or red light. After addition of DEAW, all operations on solutionsand their resulting coatings were performed under red light only. Tofurther protect them from actinic light, all solutions were prepared andstored in amber bottles. Solutions were prepared by adding components tothe solvent and mixing with a mechanical stirrer until they completelydissolved. Except for TDA and POEA, which were chromatographed onaluminum oxide (activity 1) prior to use, all components were used asreceived from the suppliers.

Solutions were coated onto a 4-mil clear film support of polyethyleneterephthalate (Mylar® polyethylene terephthalate film) at a speed of 8ft/min using a Talboy coater equipped with a 8 mil doctor knife, 12 ftdrier set at 40°-50° C., and a laminator station. A cover sheet 0.9 milpolyethylene terephthalate was laminated to the coatings as they emergedfrom the drier. Coating samples were stored in black polyethylene bagsat room temperature until used.

Sample Evaluation

Coated film was cut into 4×5-inch sections, the cover sheet was removed,and the film then mounted by laminating the tacky coating directly to aglass plate with a hand-held roller. The 4-mil polyethyleneterephthalate film support was left in place during exposure andhandling.

Coatings mounted on glass plates were evaluated by recording holographicmirrors and determining their reflection efficiency as a function ofwavelength. The mirrors were formed by actinic exposure at theintersection of two counterpropagating beams of an argon-ion laseroperating at 488 nanometers, TEM₀₀ mode (see FIG. 1). The beam intensityratio was maintained at 1:1, with absolute intensities ranging from 2-4mW/cm² per beam. Beam diameter was about one centimeter. Exposure timesranged from 30 sec to several minutes, corresponding to 150-1000 mJ/cm²total exposure. About one minute after imagewise exposure, eachholographic mirror was given a 30-60 sec fixing exposure using one ofthe two 488 nm beams. The plate was then overall exposed to ultravioletand visible light using the output of a Theimer-Strahler #5027mercury-arc photopolymer lamp (Exposure Systems Corp., Bridgeport, CT)mounted in a Douthitt DCOP-X (Douthitt Corp., Detroit, MI) exposureunit. The coating and attached film support was then removed foranalysis. Transmission spectra of the holographic mirrors were recordedfrom 400-550 nm on a Hitachi Perkin-Elmer model 330 spectrophotometer.Maximum reflection efficiency was measured from the transmissionspectra. Coating thickness was determined from photocured samples usingeither a Sloan Dektak 3030 surface profile monitoring system or a Brownand Sharpe model 1020 Electronic Comparator.

Coatings mounted on glass plates were also evaluated by recordingdiffraction gratings and determining their diffraction efficiency. Theglass mounted photopolymerizable layer were evaluated in the 30°holographic grating system described in FIG. 2. The beam intensity ratiowas maintained at about 1:1, with absolute intensities of each beamranging from 3-10 mW/cm² per beam. The diameter of each beam was aboutabout 1 cm. The photopolymerizable layer was exposed for about 4-8seconds to the modulated laser radiation corresponding to about 50-100mJ/cm² total exposure. About one minute after this image-wise exposurethe grating was reexposed for 1-2 min using one of the two emergingbeams to fix or complete polymerization throughout thephotopolymerizable layer. Grating formation was monitored using thenon-actinic 632.8 nm beam of a He:Ne laser and a detector which is aCoherent model 212 power meter attached to a strip chart recorder.Diffraction efficiency (n) is calculated as the ratio of the diffractedbeam intensity (I_(diff)) to the pre-exposure undiffracted beamintensity (I_(o)) after passing through the coating:

    n=I.sub.diff /I.sub.o                                      (1)

The samples were exposed to ultraviolet and visible radiation andcoating thicknesses measured as described above.

EXAMPLE 1

Synthesis of EAVC

A 500 mL three-necked round-bottomed flask equipped with an additionfunnel and magnetic stirrer was charged with 150 mL of anhydrousethanol. Sodium (6.45 gm, 280 mmol) was added in small pieces withstirring. When all the sodium had dissolved, ethyl acetoacetate (18.2gm, 140 mmol) and 10 mL of ethanol were added dropwise. After stirringfor a few minutes, a slurry of 1,4-dibromobutene (30.0 gm, 140 mmol) in10 mL of ethanol was added over about 1 min. An exothermic reactionensued with the precipitation of a white solid. The resulting whitesuspension was stirred at room temperature for 24 hr. The reactionmixture was filtered to remove the precipitated sodium bromide and thebulk of the ethanol removed by rotary evaporation. The resulting slurrywas taken up with 200 mL of ether and filtered again. The ether wasremoved by rotary evaporation and the product was purified by vacuumdistillation. The fraction with a bp of 52°-58° C. @ 0.5 mmHg wascollected to give 12.8 gm (51% yield) of EVAC as a clear colorless oil.Proton NMR indicated a 60:40 mixture of stereoisomers. Refractiveindex=1.4654.

EXAMPLE 2

Synthesis of EBVC

To a 2 L three-necked round-bottomed flask equipped with an additionfunnel and magnetic stirrer was added a slurry of 1,4-dibromobutene(106.0 gm, 500 mmol) in 200 mL of anhydrous ethanol. A solution ofsodium ethyl benzoylacetate, prepared by the addition of ethylbenzoylacetate (96.0 gm, 500 mmol) to a solution of sodium ethoxide inethanol (11.5 gm of sodium dissolved in 300 mL of ethanol), was addedover 0.5 hr. An exothermic reaction ensued with the precipitation ofwhite solid. After stirring for 10 min, a solution of sodium ethoxide(11.5 gm of sodium dissolved in 220 mL of ethanol) was added over 15min. The resulting white suspension was stirred for 4 hr and allowed tostand at room temperature overnight. The reaction mixture was filteredto remove the precipitated sodium bromide and the bulk of the ethanolremoved by rotary evaporation. The resulting slurry was taken up with200 mL of ether and filtered again. The ether was removed by rotaryevaporation and the product was purified by vacuum distillation. Thefraction with a bp of 95°-100° C. @ 0.04-0.01 mmHg was collected to give69.3 gm (57% yield) of EVBC as a clear colorless oil. Proton nmrindicated a 50:50 mixture of stereoisomers. Refractive index=1.5313.

CONTROL EXAMPLES A-C

The control compositions contain TDA or POEA acrylic monomers in eitherPS-MMA, PVB, or CAB binder. Formulations A-C were prepared, coated,exposed to form holographic mirrors and gratings, and analyzed asdescribed in the general procedures above. Example A is a binderorientated system; Examples B and C are monomer orientated systems.Results are given in the following table.

    ______________________________________                                        INGREDIENT (gm)   A        B        C                                         ______________________________________                                        Dichloromethane   75.0     72.0     36.0                                      Methanol          --       8.0      4.0                                       PS-MMA            15.0     --       --                                        PVB               --       10.19    --                                        CAB               --       --       5.09                                       -o-Cl HABI       0.50     0.40     0.20                                      MBO               0.50     --       --                                        MMT               --       0.40     0.20                                      TDA               9.00     --       --                                        POEA              --       7.00     3.50                                      TDC               --       2.00     1.00                                      DEAW              0.025    0.010    0.0060                                    BHT               0.0025   0.002    0.001                                     DRY COATING       44       24       22                                        THICKNESS, Microns                                                            REFLECTION        8.3%     10.6%    3.9%                                      EFFICIENCY, Maximum                                                           DIFFRACTION       93.3%    95.5%    89.3%                                     EFFICIENCY, Maximum                                                           ______________________________________                                    

EXAMPLE 3

This is a binder oriented system containing EVCD monomer and PS-MMAbinder.

The composition was prepared, coated, exposed to form a holographicmirror and a holographic transmission grating, and analyzed as describedabove. Results are given in the following table. Although holographictransmission gratings prepared from this composition and from thecomposition in Control Example A had comparable diffractionefficiencies, the reflection hologram made with this composition hadgreater reflection efficiency than the one prepared from in ControlExample A.

EXAMPLES 4-5

These are monomer orientated systems containing EBVC monomer with eitherPVB or CAB binder.

Formulations 4 and 5 were prepared, coated, exposed to form holographicmirrors and transmission gratings, and analyzed as described above.Results are given in the following table. Although diffraction gratingsprepared from these compositions were less efficient than those preparedfrom control compositions B and C, respectively, reflection hologramsmade with these compositions have greater reflection efficiency thanholograms prepared from the control compositions.

EXAMPLE 6

This is a binder orientated system containing EAVC monomer and PS-MMAbinder.

The composition was prepared, coated, exposed to form a holographicmirror and a holographic transmission grating, and analyzed as describedabove. Results are given in the following table.

EXAMPLE 7

This is a binder orientated system containing EVCD monomer and PS-ANbinder.

The composition was prepared, coated, exposed to form a holographicmirror and a holographic transmission grating, and analyzed as describedabove. Results are given in the following table.

                  TABLE 2                                                         ______________________________________                                        INGREDIENT (gm)                                                                            3       4       5     6     7                                    ______________________________________                                        Dichloromethane                                                                            75.0    72.0    72.0  37.5  75.0                                 Methanol     --      8.0     8.0   --    --                                   PS-MMA       15.0    --      --    7.50  --                                   PS-AN        --      --      --    --    16.5                                 PVB          --      10.19   --    --    --                                   CAB          --      --      10.19 --    --                                    -o-Cl HABI  0.50    0.40    0.40  0.25  1.0                                  MBO          0.50    --      --    0.25  0.50                                 MMT          --      0.40    0.40  --    --                                   EVCD         9.00    --      --    --    7.5                                  EBVC         --      7.00    7.00  --    --                                   EAVD         --      --      --    4.50  --                                   TDC          --      2.00    2.00  --    --                                   DEAW         0.025   0.010   0.012 0.00625                                                                             0.0125                               BHT          0.0025  0.0020  0.0020                                                                              0.00125                                                                             0.00125                              DRY COATING  29      25      24    27    32                                   THICKNESS,                                                                    Microns                                                                       REFLECTION   25.5%   31.2%   7.9%  4.9%  23.1%                                EFFICIENCY,                                                                   Maximum                                                                       DIFFRACTION  89.2%   57.0%   6.5%  13.3% 53.0%                                EFFICIENCY,                                                                   Maximum                                                                       ______________________________________                                    

Having described the invention, we claim:
 1. In a process for forming avolume reflection hologram by directing a reference beam and an objectbeam of coherent actinic radiation onto opposite sides of a recordingmedium, the improvement wherein the recording medium comprises asubstrate that supports a substantially solid, photopolymerizablecomposition consisting essentially of:(a) 25 to 75% of a solventsoluble, thermoplastic polymeric binder; (b) 5 to 60% of a monomercapable of addition polymerization, said monomer having a boiling pointabove 100° C. and polymerizing via free radical ring-opening sigma-bondcleavage; (c) up to 25% of a plasticizer; and (d) 0.1 to 10% of aphotoinitiator system that activates free radical polymerization of saidmonomer on exposure to actinic radiation;wherein said percentages areweight percentages of the total composition.
 2. The process of claim 1wherein said monomer contains a ring selected from the group consistingof carbocyclic rings of up to seven carbon atoms and heterocyclic ringsof up to seven atoms containing up to two heteroatoms selected from thegroup consisting of nitrogen, oxygen, and sulfur.
 3. The process ofclaim 1 wherein said monomer contains a heterocyclic ring of up to fiveatoms containing up to two heteroatoms selected from the groupconsisting of nitrogen, oxygen, and sulfur.
 4. The process of claim 1wherein said monomer is a vinylcyclopropane.
 5. The process of claim 4wherein said vinyl group is unsubstituted and at least oneelectron-withdrawing group is attached to the cyclopropane ring.
 6. Theprocess of claim 4 wherein said monomer is selected from the groupconsisting of diethyl 2-vinylcyclopropane-1,1-dicarboxylate; ethyl1-acetyl-2-vinyl-1-cyclopropane carboxylate; andethyl-1-benzoyl-2-vinyl-1-cyclopropane carboxylate; and mixturesthereof.
 7. The process of claim 1 wherein the composition has areflection efficiency of at least approximately 10% after imaging andthe coating components are present in the following approximate amounts:binder, 45 to 65%; monomer, 15 to 50%; plasticizer, 5 to 15%; andinitiator, 1 to 5%.
 8. The process of claim 7 wherein the binder ormonomer contains one or more moieties selected from the group consistingof substituted or unsubstituted phenyl, phenoxy, naphthyl, naphthyloxy,heteroaromatic groups containing up to three aromatic rings, chlorine,and bromine, and the other is substantially free of said moieties. 9.The process of claim 8 wherein said monomer contains said moiety, andsaid binder is substantially free of said moiety.
 10. The process ofclaim 9 wherein the monomer is a substituted vinylcyclopropane.
 11. Theprocess of claim 10 wherein the binder is selected from the groupconsisting of cellulose acetate butyrate polymers, acrylic polymers andinterpolymers, methyl methacrylate polymers and copolymers, polyvinylacetate, polyvinyl acetal, polyvinyl butyral, polyvinyl formal, andmixtures thereof.
 12. The process of claim 8 wherein said bindercontains said moiety, and said monomer is substantially free of saidmoiety.
 13. The process of claim 12 wherein said monomer is avinylcyclopropane.
 14. The process of claim 13 wherein the binder isselected from the group consisting of polystyrene,poly(styrene/acrylonitrile), poly(styrene/methyl methacrylate),polyvinyl benzal, and mixtures thereof.